FIG. 1 schematically illustrates a typical Soft Permanent Virtual Circuit (SPVC) 10 which includes a Permanent Virtual Circuit (PVC) connection leg 12 and a Switched Virtual Circuit (SVC) connection leg 14. For example, as shown in FIG. 1, the PVC connection leg 12 is set up from an edge router 16 to a network switch 18, and the SVC connection leg 14 is set up from the network switch 18 to another network switch 20 via a communications network 22. The SVC connection leg 14 may be set up inside a Service Provider network. As shown in FIG. 1, the network switch 20 may be an destination end device, or may be coupled to a destination device (router) 24.
An SVC is dynamically established on demand and is torn down when transmission is complete. SVCs are also referred to as Switched Virtual Connections in Asynchronous Transfer Mode (ATM) terminology. An SPVC allows devices (for example, the routers 16 and 24 in FIG. 1) attached to the network switches to be interconnected via virtual circuits which will be automatically rerouted around failures in the communications network. For example, as shown in FIG. 1, an SVC is dynamically created, using link A or link B. If either link fails, the SVC is automatically reestablished using the other link.
On the other hand, a PVC must be explicitly configured using a specific link, for example, either link A or link B in FIG. 1, and all network devices (switches and/or routers) along the virtual circuit (or path) must be explicitly configured. Each link between the network devices is also individually identified using a connection identifier, for example, virtual channel identifier (VCI) and virtual path identifier (VPI) defined at the interfaces of the network devices. In an ATM cell header, the VCI and VPI are a 16-bit field and a 8-bit field, respectively. In PVCs, the VCI and the VPI are used to identify the next destination of a cell as it passes through a series of ATM switches on its way to its destination. ATM switches use the VPI/VCI fields to identify the next network virtual channel link (VCL) that a cell needs to transit on its way to its final destination.
In a Frame Relay (FR) network, a data-link connection identifier (DLCI) is used. A value of the DLCI specifies a PVC or an SVC in a similar manner as the VPI and VCI. In the basic FR specification, DLCIs are locally significant (connected devices might use different values to specify the same connection). However, in the Local Management Interface (LMI) extended specification, DLCIs are globally significant and DLCIs specify individual end devices.
A connection service category is associated with each SPVC connection, which is typically defined by traffic parameters and parameter values per interface. The association between an SPVC connection and its service category is set up at the time of the creation of the connection. If the service category of an existing SPVC connection needs to be changed, a user or system administrator must delete the entire SPVC connection, and then create a new connection for the new service category with the modified/new traffic parameters. That is, both of the PVC and SVC legs must be removed and recreated with new commands using (hopefully) the same VPI/VCI values. However, since there is always a finite delay (SPVC downtime) between an explicit deletion of the existing connection and reconfiguration of a new connection, the VPI/VCI values in the original connection might be used by other circuits during the downtime, and thus the original VPI/VCI values cannot be guaranteed in the new connection. This means that the new connection would lose its identity with the original connection, and would be considered as a completely different connection.
This may pose a problem especially at the edge where the Service Provider may require the Enterprise or connecting router at the edge to change the parameters. The problem becomes complicated, if the connecting router/switch is in a restricted domain such as a Central Office since some legal paperwork may have to be executed for a new connection identified by new VPI/VCI values different form the original values.